1. Field of the Invention
The present invention relates generally to a WDM (Wavelength Division Multiplexed) PON (Passive Optical Network) using loop-back light sources. More particularly, the present invention relates to an optical wavelength tracking apparatus and method for rendering the WDM wavelengths of a multi-frequency light source in a central office (CO) identical to those of a WDM multiplexer/demultiplexer (MUX/DEMUX) in a remote node (RN).
2. Description of the Related Art
A WDM PON provides very high-speed, wide-band communication service to subscribers by optical signals having different subscriber-specific wavelengths. Thus, the WDM PON helps to ensure the privacy of communications, and easily satisfies individual users' demands for additional communication services or a larger communication capacity. This service accommodates more and more subscribers simply by adding specific wavelengths for new subscribers. Despite these advantages, the WDM PON has serious drawbacks that have delayed deployment because of the requirement of an additional wavelength stabilizing circuit for stabilizing the oscillation frequency of a light source that generates an optical signal in both a CO and each subscriber device. Therefore, the development of an economical WDM light source is essential to implementation of the WDM PON. Loop-back light sources, such as a Fabry-Perot laser injection-locked to the wavelength of an external optical signal or a reflective semiconductor optical amplifier (RSOA), have been proposed as economical WDM light sources for upstream data transmission from an optical network unit (ONU) as a subscriber device to a CO. A loop-back light source used for upstream data transmission in the ONU receives an optical signal from the CO and outputs an optical signal having the same wavelength as that of the received optical signal that is modulated according to the upstream transmission data. Hence, the loop-back light source does not need frequency selection and wavelength stabilization. In addition, the Fabry-Perot laser is a low-price light source that is popular because the laser outputs an optical signal injection-locked to an input optical signal and the injection-locked signal is a high-power signal with a narrow linewidth. Therefore, the Fabry-Perot laser can transmit modulated data at high speed. The RSOA amplifies an input optical signal to a high power level even though it is at a very low power level, and modulates the amplified signal according to an upstream transmission data signal. Hence, if the ROSA is used as a Loop-back light source in the ONU, a low price multi-frequency light source, which generates a WDM optical signal destined for loop-back light source in the ONU, is usable in the CO.
In general, the physical configuration of the PON is that of a double star topology, thereby minimizing the length of the optical lines used. In other words, the CO is connected to an RN close-by to subscribers by a single optical fiber, and the RN is in turn connected to the individual subscribers by independent optical fibers. Therefore, the CO and the RN are provided with a WDM MUX for WDM-multiplexing optical signals having different wavelengths and a WDM DEMUX for WDM-demultiplexing the WDM optical signal. Arrayed waveguide gratings (AWGs) are usually used as the WDM MUX/DEMUX. The RN close-by to the subscribers does not require a device to maintain an internal temperature constant. As a result, the RN is affected by temperature changes between seasons, or between day and night. The WDM wavelengths of the AWG WDM MUX/DEMUX in the RN are subsequently changed by the temperature variations. The wavelength variation of the AWG with temperature is determined according to the material of the AWG. If the AWG is formed of an III-IV group compound, a typical semiconductor material, its wavelength variation with temperature is about 0.1 nm/° C. If the AWG is formed of silica (SiO2), the wavelength variation with temperature is about 0.01 nm/° C.
As the WDM wavelengths of the WDM MUX/DEMUX in the RN change with corresponding temperature changes, the WDM wavelengths of the multi-frequency light source in the CO are not exactly the same as compared with those of the WDM MUX/DEMUX in the RN. Similarly, the WDM wavelengths of the WDM DEMUX in the CO are at variance from those of the WDM MUX/DEMUX in the RN. The resulting increase in the output loss of upstream and downstream channels and crosstalk from adjacent channels degrades system transmission performance. Accordingly, there is a need for developing optical wavelength tracking techniques for making the WDM wavelengths of the multi-frequency light source and the WDM DEMUX identical to those of the WDM MUX/DEMUX in the RN in order to prevent the transmission performance degradation caused by the temperature change in the RN.
In light of the above, there have been proposed some optical wavelength tracking schemes to render the wavelengths of the WDM light source for downstream transmission to be identical to those of the AWG varying with temperature in the RN in the WDM PON. One of them is U.S. Pat. No. 5,729,369 entitled “Method of Tracking a Plurality of Discrete Wavelengths of a Multi-Frequency Optical Signal for Use in a Passive Optical Network Telecommunications System”, invented by Martin Zirngibl, and granted on Mar. 17, 1998. According to the patent, the discrete wavelengths are tracked by removing one of ONUs connected to a multi-frequency router corresponding to the WDM MUX/DEMUX in the RN and reflecting back a wavelength corresponding to the removed ONU in the upstream direction.
Another optical wavelength tracking scheme attempting to solve the aforementioned problems is found in U.S. Pat. No. 6,304,350 entitled “Temperature Compensated Multi-Channel Wavelength-Division-Multiplexed Passive Optical Network”, invented by Christopher Richard Doerr, et. al., and granted on Oct. 16, 2001. In this disclosure, the RN detects the power level of one of channels at a WGR (Waveguide Grating Router) corresponding to the WDM MUX/DEMUX and notifies the CO of the detected power level. The CO then changes the temperature of a multi-frequency laser (MFL) (which is a multi-frequency light source) in accordance with the change of the received power level, to thereby enable the frequencies of the MFL in the CO to track the channels of the WGR in the RN.
The above optical wavelength tracking techniques are inefficient because one of WDM wavelengths is only used for tracking, and these techniques require an additional device by which the RN measures the power level of one channel and transmits the power level to the CO, thereby increasing cost.